The properties of polymers depend upon the properties of the catalyst used in their preparation. In catalysts, control of the shapes, sizes, and the size distribution of the catalyst is important to ensure a good commercial workability. This is particularly important in gas phase and slurry polymerization. For example, in order to produce copolymer granules of 1000 μm in size, a catalyst particle size of about 10 μm to about 50 μm is generally preferred for use in the polymerization. A catalyst should have good mechanical properties to resist wear during the polymerization process and to ensure a good bulk density of the polymer produced. One important aspect relating to the development of a polymerization catalyst is, therefore, the provision of a catalyst and process for production of a catalyst that allow control and adjustment of the structures and sizes of the catalyst particles and particle size distribution. Preparation of such catalyst should remain a necessarily simple process.
Spray-drying is one technique for preparing catalyst particles that allows control of the size and shape of resulting catalysts. In spray-drying, liquid droplets containing dissolved and/or suspended materials are ejected from a flywheel or a nozzle. The solvent evaporates leaving behind a solid residue. The resulting particle size and shape is related to the characteristics of the droplets formed in the spraying process. Structural reorganization of the particle can be influenced by changes in volume and size of the droplets. Depending on conditions of the spray drying process, either large, small, or aggregated particles may be obtained. The conditions may also produce particles that are compositionally uniform or mixtures of solution components. The use of inert fillers in spray-drying can help control shape and composition of the particles.
Numerous spray-dried olefin polymerization catalysts containing magnesium and titanium and production processes utilizing them have been reported. However, the magnesium content in these processes is limited by the solubility of the magnesium in the solvent. Generally, solubility is expected to increase with temperature. However, the solubility of magnesium halides in some preferred organic solvents, such as tetrahydrofuran (THF), in which the magnesium-containing components are dissolved actually decreases from about room temperature to the boiling point of such solvents. The reduced solubility is thought to result from the formation of polymeric magnesium halide-solvent complexes with lower solubility, such as MgCl2(THF)1 5-2. For example, the maximum concentration of ultra-pure magnesium chloride that can be achieved in THF is less than about 0.75 moles MgCl2/liter. At about 60° C., near the boiling point of THF, the solubility of the magnesium chloride is noticeably decreased to less than 0.5 moles/liter. However, when commercial grade magnesium chloride is used, its maximum solubility in THF is lowered to about 0.6 moles MgCl2/liter. The solubility of magnesium chloride in solutions derived from commercial grade magnesium chloride is only about 0.35 moles/liter at 60° C.
The low solubility of magnesium in the solvent limits the amount and distribution of magnesium halide that can be incorporated into a spray-dried catalyst particle. However, high concentrations of magnesium in the spray-dried particles provide catalysts that produce polymers with more desirable properties and increased catalytic activity; thereby, increasing the catalyst's desirability and cost-effectiveness. Thus, providing a spray-dried catalyst that has increased magnesium content would be desirable.